It is often desirable to convert raw hydrocarbon mixtures such as crude oil and other petroleum feedstocks to commercially valuable fuels. A number of processes for cracking hydrocarbons are known. These processes include, for example, thermophor catalytic cracking (TCC) and fluid catalytic cracking (FCC) (including the FCC process of Ashland/UOP known as reduced crude conversion (RCC)). These processes are described in the literature; reference for TCC: Gary and Handwerk, "Petroleum Refining - Technology and Economics", Marcel Dekker, Inc., 1975, pg. 90-95. Reference for FCC: Venuto and Habib, "Fluid Catalytic Cracking with Zeolite Catalysts", Marcel Dekker, Inc., 1979. Reference for RCC: Lomas, Hammershaimb, and Yunoki, "RCC Technology for New Installations and FCC Unit Conversion", Japan Petroleum Institute Refining Symposium, 1984.
The cracking of hydrocarbons is accomplished by contacting the hydrocarbon to be cracked with a catalyst at elevated temperatures. The catalysts most commonly used for cracking hydrocarbons comprise a crystalline aluminosilicate zeolite that has been incorporated into a matrix. These zeolites are well known and have been described, for example, in U.S. Pat. Nos. 4,432,890, 4,707,461 and 4,465,779.
The matrix into which the zeolite is incorporated may be natural or synthetic, and typically has substantially less catalytic activity relative to the zeolite component. Some suitable matrices include clays, silica, and/or metal oxides such alumina and mixtures thereof.
A major difficulty with cracking catalysts is their tendency to become deactivated following contact with certain heavy metal contaminants present in the hydrocarbon feedstock. The deleterious metals include vanadium, nickel, iron, copper and sodium, with vanadium being considered the most deleterious. These metals may be present in the hydrocarbon as free metals or as components of inorganic and organic compounds such as porphyrins and asphaltenes. In addition to lost activity, the catalyst becomes less selective, resulting in increased amounts of undesirable products such as coke and light gases, i.e., hydrogen, methane and ethane.
The magnitude of catalyst deactivation by metals in the feed is known. For example, catalyst requirements as high as 0.5-1.0 lbs per barrel of feed are typical when processing resids compared with 0.1-0.2 lb of catalyst per barrel for conventional gas oils. This dramatic increase in catalyst usage is largely due to the higher metals content of the heavier resid feeds.
The deleterious effects of metals have been discussed extensively in the literature, for example in U.S. Pat. Nos. 4,376,696; 4,513,093; 4,515,900, and are generally well known to those skilled in the art. Vanadium is known to substantially deactivate cracking catalysts by irreversibly destroying the active zeolite. Nickel, iron and copper promote dehydrogenation reactions which result in increased coke and dry gas yields at the expense of the desired liquid products. Sodium reduces catalyst activity by neutralizing acid sites and promoting zeolite degradation.
Methods for counteracting the deleterious effects of heavy metals have been developed. For example, it is known to treat hydrocarbon feeds containing such metal contaminants with a variety of other metals that are said to passivate the contaminating metals. These metals may be added to the hydrocarbon feed as the free metal, or as salts or compounds of the metal, for example, the metal oxide or an organometallic compound. It is believed that the passivating metals form complexes with the contaminating metals, and that the complexes are less harmful to the cracking catalysts than are the uncomplexed contaminating metals.
For example, Beck et al., U.S. Pat. No. 4,432,890 (Ashland Oil, Inc.) discloses the addition of metals such as titanium, zirconium, manganese, indium and lanthanum to a cracking unit during the cracking process. The metals or their oxides or salts may be added to the cracking unit incorporated into a catalyst matrix. Alternatively, soluble compounds of the metals such as organometallic compounds may be added to the cracking unit along with the catalyst and its matrix.
As a further example, Mitchell et al., U.S. Pat. No. 4,707,461 (Chevron Research Company) discloses the addition of a calcium additive such as calcium carbonate during catalytic cracking. The calcium additive may be part of the catalyst matrix or may be introduced separately from the catalyst matrix.
Similarly, Occelli et al., U.S. Pat. No. 4,465,779, (Gulf Research & Development Company) discloses metal additives for passivating contaminating metals in hydrocarbon feeds. The metal additives are magnesium compounds optionally in combination with a heat-stable metal compound such as an oxide of silicon, aluminum, iron, boron, zirconium, phosphorus and certain clay minerals. The additives described by Occelli et al. are separate and distinct from the catalyst.
While methods such as those described above for passivating metal contaminants in hydrocarbon feedstocks have been helpful, the recent trend toward the refining of heavier feedstocks containing larger amounts of metal contaminants require still better passivating methods.
Accordingly, an objective of the present invention is to provide an improved method for passivating metal contaminants such as vanadium, nickel, iron, copper and sodium, and especially vanadium, in hydrocarbon feedstocks during the cracking process. It is a further objective of the present invention to provide a method for passivating metals during catalytic cracking by contacting the metals with a precipitate of a rare earth oxide such as lanthanum oxide, alumina and aluminum phosphate (hereinafter, REOAAP and LAAP, respectively).